This section provides background information related to the present disclosure which is not necessarily prior art.
Motors are used to power the operation of a wide range of devices from very small scale machines to much larger assemblies such as elevators (also known as “lifts”). One of the most common forms of motor used, particularly in industrial applications requiring a constant motor speed, are synchronous motors. Synchronous motors synchronise the rotation of a shaft of a motor with a frequency of an Alternating Current (AC) electrical supply used to power the motor.
In general terms, a synchronous motor comprises a stator and a rotor. The stator includes a number of coils or windings through which electric currents can be fed. The rotor comprises at least one pair of magnets which may be electromagnets or permanent magnets. When an AC current is fed through a winding of the stator, the winding generates a changing magnetic field. Therefore, in a three-phase motor, when the three-phase components of a three-phase AC current are fed through three respective windings, a rotating magnetic field is created in the stator. The rotating magnetic field created in the stator causes rotation of the rotor and, when the load driven by the motor is low, the speed of rotation of the rotor is synchronous with the frequency of the three-phase AC current. The angle between the rotor and the stator produces a resultant net torque, which dictates the net rotational movement of the rotor.
In order for the net rotational movement of the rotor to be in a desired direction and at a desired speed at any given time, the net torque of the rotor must be controlled. The position and phase at which current is injected into the windings of the stator relative to the magnets of the rotor will determine the configuration of the magnetic flux produced by the stator. This will affect the rotational movement imparted by the winding on the rotor, which in turn determines the net torque exerted upon the rotor.
In order to determine and control the torque produced by a motor, the relative position between the stator and the rotor should be determined. Only current that has a component perpendicular to the rotor's magnetic flux will cause torque to be exerted upon the rotor. For a stator which has three windings, the current vector resulting from the three currents in combination should be perpendicular to the rotor's magnetic flux in order to maximise torque; if instead the current vector and the magnetic flux are parallel, then no torque will be produced. Therefore, maximum torque can be achieved for a synchronous motor by controlling the phase and timing of currents supplied to the stator.
A detector such as a rotary (or shaft) encoder may be used to provide information on the position of a rotor relative to a stator. A rotary encoder encodes information about the relative angular position or motion of a shaft in an analogue or digital manner. One type of rotary encoder is an incremental rotary encoder, which provides an output only when the encoder is rotated. Another type is an absolute encoder, which produces a unique code for each distinct angular position of the shaft.
Rotary encoders can comprise a rotatable disc having a plurality of apertures and being mounted on the shaft of the rotor and one or more light emitting devices (LEDs) and photodetectors coupled to the stator. Light emitted from the LED(s) is received by a given photodetector only when a corresponding aperture in the disc is aligned with that photodetector. Accordingly, rotation of the shaft will cause the photodetectors to receive a series of signals from which the relative positioning between a point on the rotor and a point on the stator can be determined.
An encoder can comprise memory on, for example, an electronic chip and a processor and/or it can provide feedback to another device that comprises a processing means. In a drive system, the controller or “drive unit” includes a processor which can receive and process information from the encoder. Primarily, an encoder operates as a feedback device to enable the drive unit to use the relative rotor and stator positions to determine the timing, phase and angle of current to be supplied to the stator windings. Encoders are not always mounted on rotor shafts in a predetermined manner and so there may be a ‘commutation offset’ angle between the actual zero position of a given encoder on a shaft and the zero position of the rotor with respect to the stator. For example, the commutation offset may be measured as the angle between the zero position of the encoder and the north pole of a magnet on the rotor.
Aspects and features of the present disclosure are set out in the appended claims.